Rheological agent, preparation methods and uses thereof

ABSTRACT

The present invention relates to a method to prepare a rheological agent comprising the polymerization of the following monomers: (a) at least one styrene-based monomer, (b) at least one (meth)acrylate-based monomer or butadiene, and (c) optionally a copolymerisable surfactant containing an optionally substituted vinyl function and moieties derived from propylene oxide and/or ethylene oxide, optionally in the presence of a non polymerisable surfactant containing moieties derived from propylene oxide and/or ethylene oxide, and optionally in the presence of another surfactant, wherein at least the copolymerisable surfactant or the non polymerisable surfactant is present during the said polymerization, as well as a rheological agent obtainable by said method, an oil-based drilling fluid containing such a rheological agent, the use of such a rheological agent as a thixotropic agent in an oil-based drilling fluid and a method of drilling using an oil-based drilling fluid containing such a rheological agent.

The present invention relates to the field of oil-based drilling fluidsused notably in drilling subterranean wells, and more particularly to arheological agent which can be used in replacement of organoclayspresent in such drilling fluids.

Drilling fluids, also called drilling muds, are used notably in drillingoil, gas and geothermal subterranean wells to maintain pressure andwellbore stability, cool, lubricate and support drill bits, controlcorrosion, and suspend and remove drill cuttings from the well duringthe drilling operation.

These drilling fluids are circulated under great pressure down a longdrilling pipe and then are returned to the surface through the pipe holeannulus, i.e. the zone between the outside of the pipe and the boreholewall. The drill cuttings generated by the drill bit are thus carried inthe drilling fluid to the surface where the drilling fluid is processedto remove these drill cuttings before being circulated again in thedrilling well.

The composition of such drilling fluids depends on the well to bedrilled. There exist thus two main classes of drilling fluids:water-based drilling fluids and oil-based drilling fluids.

Oil-based systems are being increasingly used in practice particularlyin offshore drilling, for the drilling of hydrogen sulfide-containingformations or for the drilling of water-sensitive layers, such as shale,salt, gypsum, anhydrite or evaporite layers.

Oil-based drilling fluids comprise generally an invert emulsion, i.e. awater-in-oil emulsion comprising an external continuous oil phase inwhich is dispersed a discontinuous internal aqueous phase.

Emulsifiers are thus generally present in such compositions to stabilisethe emulsion. Other additives well known to the person skilled in theart can also be present, such as weighting agents, wetting agents, fluidloss control additives, alkalinity regulators, viscosity regulators andthe like, for stabilising the system as a whole and for establishing thedesired performance properties. Full particulars can be found, forexample, in the article of P. A. Boyd et al. Journal of PetroleumTechnology 1985, 131-142 and in the article of R. B. Bennet et al.Journal of Petroleum Technology 1984, 875-981.

An important property of drilling fluids is notably to have a lowviscosity at high shear and a higher viscosity upon standing to formquickly a thick gel when the drilling operations are stopped in order tomaintain cuttings in suspension, even at high temperatures since thetemperature of the drilling bottom hole can reach 300° F. (about 149°C.) or more, but also to maintain the homogeneity of the drilling fluidduring its transport to the drilling site. Therefore, to maintain thedesired rheology and viscosity of the drilling fluids, rheologymodifiers and viscosifiers are added in the drilling fluids, an inparticular organoclays (also called organophilic clays which areorganically modified clays by treatment with organocations to form anorganophilic surface).

However, organoclays are added to the drilling fluids in the form ofsolid particles, which reduces the amount of drill cuttings that thefluid can tolerate thereby reducing the lifetime of the drilling fluidbefore treatment and can also cause formation damage by plugging porethroats thereby reducing permeability and production. The solid form oforganoclays represents also a problem during transport and storage sincethere is a risk of settling out of the organoclays during these periods.It is thus generally necessary to keep the fluid circulating or torender it very thick by adding permanent or temporary viscosifiers tomaintain the homogeneity of the drilling fluid. Moreover a thermaldegradation of organoclays can also occur at high temperature leading tothe loss of the desired rheology of the drilling fluid.

There is thus a need to replace organoclays in oil-based drilling fluidswith other rheology modifiers to maintain the desired property of theoil-based drilling fluids while avoiding the above-mentioned drawbacksof organoclays.

The inventors of the present patent application have thus discoveredthat it was possible to replace the organoclays present in oil-baseddrilling fluids with a rheological agent made of a polymer comprising ahydrophobic backbone soluble in the oil phase of the oil-based drillingfluids, as well as hydrophilic functions, linked or not to the saidhydrophobic backbone polymer.

Indeed the rheological agent according to the invention has thefollowing properties and advantages:

-   -   complete dissolution of the rheological agent in oil-based        drilling fluids,    -   chemical and physical stability on a large range of        temperatures,    -   thixotropic properties, i.e. the rheological agent forms a gel        upon standing and has a low viscosity under high shear, allowing        control of the rheology of the drilling fluid during transport        and storage phase, as well as before and during the use of the        drilling fluid in the drilling well, and    -   better control of the rheological profile of the drilling fluid,        such as a lower plastic viscosity.

A rheological agent with such properties and advantages is obtained byadding a surfactant, polymerisable or not, to the mixture of monomerswhich will lead to the hydrophobic backbone of the polymer, during thepolymerization step.

The present invention concerns thus a method to prepare a rheologicalagent comprising the polymerization of the following monomers:

-   -   (a) at least one styrene-based monomer,    -   (b) optionally at least one (meth)acrylate-based monomer or        butadiene, and    -   (c) optionally a copolymerisable surfactant containing an        optionally substituted vinyl function and moieties derived from        propylene oxide and/or ethylene oxide,        optionally in the presence of a non polymerisable surfactant        containing moieties derived from propylene oxide and/or ethylene        oxide, and        optionally in the presence of another surfactant,        wherein at least the copolymerisable surfactant or the non        polymerisable surfactant is present during the said        polymerization.

It should be noted that, when a non polymerisable surfactant is present,it is important that this non polymerisable surfactant is present duringthe polymerization step, even if it does not react with the overmonomers, since the simple admixture of a polymer obtained bypolymerization of monomers (a) and (b) with the said non polymerisablesurfactant does not give a compound having the desired properties.

The terms “(C₁-C₂₅)alkyl”, “(C₁-C₁₅)alkyl” and “(C₁-C₆)alkyl”, as usedin the present invention, refer to a straight or branched monovalentsaturated hydrocarbon chain containing respectively 1 to 25, 1 to 15 or1 to 6 carbon atoms including, but not limited to, methyl, ethyl,n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, t-butyl, n-pentyl,n-hexyl, dodecyl and the like.

The term “(C₁-C₆)alkanediyl”, as used in the present invention, refersto a straight or branched divalent saturated hydrocarbon chaincontaining from 1 to 6 carbon atoms including, but not limited to,methanediyl, ethanediyl, propanediyl, butanediyl, pentanediyl,hexanediyle, and the like.

The term “(C₂-C₆)alkenyl”, as used in the present invention, refers to astraight or branched unsaturated hydrocarbon chain containing from 2 to6 carbon atoms and comprising at least one double bond including, butnot limited to, vinyl, propenyl, butenyl, pentenyl, hexenyl and thelike. It is in particular a vinyl group.

The term “(C₅-C₈)cycloalkyl”, as used in the present invention, refersto a saturated hydrocarbon ring having 5 to 8 carbon atoms including,but not limited to, cyclopentyl, cyclohexyl and the like.

The term “aryl”, as used in the present invention, refers to an aromaticgroup comprising preferably 6 to 10 carbon atoms and comprising one ormore fused rings, such as, for example, a phenyl or naphtyl group.Advantageously, it will be a phenyl group.

The term “aryl-(C₁-C₆)alkyl”, as used in the present invention, refersto an aryl group as defined above bound to the molecule via a(C₁-C₆)alkyl group as defined above. In particular, an aryl-(C₁-C₆)alkylgroup is a benzyl or phenylbutyl group.

The term “optionally substituted vinyl function”, as used in the presentinvention, refers to a group —CHR═CHR′ in which R and R′ represent,independently of each other, a hydrogen atom or a substituent such as a(C₁-C₂₅)alkyl, in particular a (C₁-C₆)alkyl, notably a methyl. It can beadvantageously a group —CHR═CH₂ with R representing a (C₁-C₆)alkyl,notably a methyl. Advantageously, it is a group —CH═CH₂ (vinyl) or—CMe=CH₂.

The term “moieties derived from propylene oxide and/or ethylene oxide”,as used in the present invention, refers to a group of the followingformula:

—(O—CHR^(1a)—CHR^(2a))_(a)—

wherein each R^(1a) and each R^(2a) represent, independently of eachother, a hydrogen atom or a methyl group but R^(1a) and R^(2a) cannotboth represent a methyl group; and a represents an integer of at least1, notably at least 3.

When the surfactant concerned is a copolymerisable surfactant, arepresents advantageously an integer of at least 3, notably comprisedbetween 3 and 120, in particular between 5 and 70, preferably between 5and 40. When the surfactant concerned is a non polymerisable surfactant,a represents advantageously an integer comprised between 1 and 60,notably between 3 and 20.

Thus the moieties —(O—CHR^(1a)—CHR^(2a))— represent moieties derivedeither from ethylene oxide (OCH₂CH₂) or from propylene oxide (OCH₂CHMeor OCHMeCH₂).

Preferably, it will be a group of the following formula:

—(O—CHR³—CHR⁴)_(b)—(O—CHR⁵—CHR⁶)_(c)—

wherein:

-   -   b and c each represents, independently of each other, an integer        above or equal to 0, with b+c≧1, notably b+c≧3,    -   R³ and R⁴ each represents, independently of each other, a        hydrogen atom or a methyl group but cannot both represent a        methyl group, and    -   R⁵ and R⁶ each represents, independently of each other, a        hydrogen atom or a methyl group but cannot both represent a        methyl group.

When the surfactant concerned is a copolymerisable surfactant, b and ceach represents advantageously, independently of each other, an integerabove or equal to 0, with b+c≧3, notably 3≦b+c≦120, in particular3≦b+c≦70, preferably 5≦b+c≦40. When the surfactant concerned is a nonpolymerisable surfactant, b and c each represents advantageously,independently of each other, an integer above or equal to 0, with b+c≧1,notably 1≦b+c≦60, preferably 3≦b+c≦20.

The moieties —(O—CHR³—CHR⁴)_(b)— and —(O—CHR⁵—CHR⁶)_(c)— represent theneither a polyethylene oxide (PEO) moiety, i.e. (OCH₂CH₂)_(x) with x=b orc, or a polypropylene oxide (PPO) moiety, i.e. (OCH₂CHMe)_(y) or(OCHMeCH₂)_(y) with y=b or c.

The term “styrene-based monomer” refers to a monomer made of a styrenemoiety optionally substituted, preferably on the phenyl moiety, inparticular in the para position, with one or more substituents. Thesubstituent can be a (C₁-C₁₅)alkyl, in particular a (C₁-C₆)alkyl, a(C₂-C₆)alkenyl, a (C₅-C₈)cycloalkyl, an aryl or an aryl-(C₁-C₆)alkyl,and in particular a (C₁-C₁₅)alkyl. Preferably, it will be a(C₁-C₆)alkyl-styrene, and notably a p-(C₁-C₆)alkyl-styrene.

The styrene-based monomer can be chosen among styrene,meta-methyl-styrene, para-methyl-styrene, para-propyl-styrene,para-tert-butyl-styrene, para-cyclohexyl-styrene, para-dodecyl-styrene,2-ethyl-4-benzyl-styrene, para-(phenylbutyl)-styrene, divinylbenzene andmixtures thereof. In particular, it can be para-tert-butyl-styrene(PTBS), para-methyl-styrene (PMS) and mixtures thereof.

Preferably, when present, monomer (b) will be at least one(meth)acrylate-based monomer.

The term “(meth)acrylate” is well known to the person skilled in the artand refers both to methacrylate and acrylate derivatives.

The term “(meth)acrylate-based monomer” refers thus to a monomer made ofa (meth)acrylate moiety, i.e. a methacrylate or acrylate moiety. This(meth)acrylate moiety can correspond to the following formula:

wherein:

-   -   R_(a) represents a hydrogen atom or a methyl group, and    -   R_(b) represents a (C₁-C₁₅), notably (C₁-C₁₀) saturated        hydrocarbon group.

Said saturated hydrocarbon group can be straight or branched, and thusrepresents a (C₁-C₁₅)alkyl, notably a (C₁-C₁₀)alkyl group.

Said saturated hydrocarbon group can also be mono- or poly-cyclic, thecycles having advantageously 5- or 6-members and being optionallysubstituted with one or more (C₁-C₄)alkyl groups.

The (meth)acrylate-based monomer can be chosen among ethyl acrylate,n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, tert-butylacrylate, pentyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, methylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, 2-ethylhexylmethacrylate, lauryl methacrylate, isobornyl methacrylate and mixturesthereof.

Monomer (b) can be notably at least one (meth)acrylate-based monomerchosen among isobornyl methacrylate (IBOMA), isobornyl acrylate (IBOA),isobutyl methacrylate (IBMA), 2-ethylhexyl acrylate (2EHA) and mixturesthereof.

It can be in particular isobornyl methacrylate (IBOMA), isobutylmethacrylate (IBMA), 2-ethylhexyl acrylate (2EHA) and mixtures thereof.It can be in particular IBOMA.

The ratio by weight of monomer (a) to monomer (b) can be comprisedbetween 100:0 and 10:90, advantageously between 100:0 and 50:50, andnotably between 100:0 and 70:30.

The term “copolymerisable surfactant” means that the surfactant containsa function which can react with a function of monomers (a) and/or (b)during the polymerization step to form a polymer, i.e. an optionallysubstituted vinyl function.

Such a copolymerisable surfactant is preferably an anionic or non-ionicsurfactant carrying an optionally substituted vinyl function andmoieties derived from propylene oxide and/or ethylene oxide. The vinylfunction is advantageously optionally substituted with a (C₁-C₂₅)alkyl,notably a (C₁-C₆)alkyl, such as a methyl. Such a surfactant can thuscorrespond to the following formula:

CHR′═CR-L-(O—CHR^(7a)—CHR^(8n))_(n)—X¹

wherein:

-   -   L represents a single bond or a linker,    -   R and R′ represent, independently of each other, H or a        (C₁-C₂₅)alkyl, in particular H or a (C₁-C₆)alkyl, notably H or a        methyl, preferably H,    -   n represents an integer of at least 1, notably comprised between        3 and 120, in particular between 5 and 70, preferably between 5        and 40,    -   each R^(7n) and each R^(8n) represent, independently of each        other, a hydrogen atom or a methyl group but R^(7n) and R^(8n)        cannot both represent a methyl group, and    -   X¹ represents —O-(optionally substituted aryl), —O(C₁-C₂₅)alkyl,        —O—(C₁-C₆)alkanediyl-SO₃H, —OH (alcohol), —S(O)₂(OH)        (sulfonate), —O—P(O)(OH)₂ (phosphate) or —O—S(O)₂(OH) (sulfate)        group, or a salt thereof, such as a sodium, potassium or        ammonium salt.

Thus the moieties —(O—CHR^(7n)—CHR^(8n))— represent moieties derivedeither from ethylene oxide (OCH₂CH₂) or from propylene oxide (OCH₂CHMeor OCHMeCH₂).

The term “linker” refers to a divalent group (generally a small group)used to link the “vinyl” moiety —CR═CHR′ to the rest of the surfactantsuch as:

(C₁-C₆)alkanediyl such as —(CH₂)_(k)— (k=1 to 6, notably 1), —C(═O)—,—O—(C₁-C₁₅)alkanediyl- such as —O—(CH₂)_(t)— (t=1 to 6), aryl optionallysubstituted with a (C₁-C₁₅)alkyl, —O-aryl- optionally substituted with a(C₁-C₁₅)alkyl,

with A=(C₁-C₁₅)alkyl or (CH₂)_(u)OA¹ with A¹ representing a hydrocarbonchain and u=1 to 6, notably 1. The linker can be also a polyoloptionally substituted.

X¹ represents advantageously —OH (alcohol), —S(O)₂(OH) (sulfonate),—O—P(O)(OH)₂ (phosphate) or —O—S(O)₂(OH) (sulfate) group, or a saltthereof, such as a sodium, potassium or ammonium salt.

Advantageously, R′═H and R═H or (C₁-C₆)alkyl, notably H or methyl,preferably H.

The aryl moiety of X¹ is preferably a phenyl. This aryl moiety can beoptionally substituted in particular with one or several groups chosenamong (C₁-C₆)alkyl, aryl and aryl-(C₁-C₆)alkyl.

The copolymerisable surfactant can correspond in particular to thefollowing formula:

CHR′═CR-L-(O—CHR⁹—CHR¹⁰)_(m)—(O—CHR¹¹—CHR¹²)_(p)—X¹

wherein:

-   -   R, R′, L and X¹ are as defined above,    -   m and p each represents, independently of each other, an integer        above or equal to 0, with m+p≧3, notably 3≦m+p≦120, in        particular 3≦m+p≦70, preferably 5≦m+p≦40,    -   R⁹ and R¹⁰ each represents, independently of each other, a        hydrogen atom or a methyl group but cannot both represent a        methyl group, and    -   R¹¹ and R¹² each represents, independently of each other, a        hydrogen atom or a methyl group but cannot both represent a        methyl group.

The moieties —(O—CHR⁹—CHR¹⁰)_(m)— and —(O—CHR¹¹—CHR¹²)_(p)— representthen either a polyethylene oxide (PEO) moiety, i.e. (OCH₂CH₂)_(x) withx=m or p, or a polypropylene oxide (PPO) moiety, i.e. (OCH₂CHMe)_(y) or(OCHMeCH₂)_(y) with y=m or p.

It can be in particular one of the following compounds:

Compound Trade name

  with R representing a hydrocarbon chain Reasoap ® SR-10 (sold byAdeka)

  with R representing a hydrocarbon chain Reasoap ® SR-20 (sold byAdeka)

  with R representing a hydrocarbon chain Reasoap ® ER-10 (sold byAdeka)

Reasoap ® PP-70 (sold by Adeka)

Emulsogen ® APS 100 (sold by Clariant)

The term “non polymerisable surfactant” means that the surfactantcontains no function which can react with monomers (a) and (b) to formthe polymer and thus that this surfactant will not be covalently linkedto the polymer formed during the polymerization step.

Such a non polymerisable surfactant is preferably an anionic ornon-ionic surfactant containing moieties derived from propylene oxideand/or ethylene oxide, and in particular derived from ethylene oxide.Such a surfactant can correspond to the following formula:

X³—(OCH₂CH₂)_(q)—X²

wherein:

-   -   q represents an integer comprised between 1 and 60, notably        between 3 and 20,    -   X³ represents a (C₈-C₂₅)alkyl chain, i.e. a straight or branched        saturated hydrocarbon chain containing 8 to 25 carbon atoms, or        an aryl, such as a phenyl, optionally substituted with a        (C1-C₂₅)alkyl, and    -   X² represents —OH (alcohol), —S(O)₂(OH) (sulfonate),        —O—P(O)(OH)₂ (phosphate) or —O—S(O)₂(OH) (sulfate) group, or a        salt thereof, such as a sodium, potassium or ammonium salt.    -   X³ is in particular a (C₈-C₂₅)alkyl chain.

It can be in particular one of the following compounds:

Compound Trade name

Emulsogen ® EPA 073 (sold by Clariant)

Emulsogen ® LA 083 (sold by Adeka)

Rhodafac ® RS-610 A25 (sold by Rhodia)

Disponil ® FES993 (sold by Cognis)

The copolymerisable surfactant and the non polymerisable surfactant haveto be present during the polymerization step.

According to a particular embodiment, the rheological agent comprises atleast one copolymerisable surfactant. Indeed, the rheological agent thusobtained has better rheological properties.

Advantageously, the rheological agent will comprise 0.5 to 10 wt %,notably 1 to 7 wt %, of copolymerisable and non polymerisablesurfactants relatively to the total weight of the rheological agent on adry basis (i.e. to the active parts of the rheological agent).

The polymerization reaction is carried out in conditions well known tothe person skilled in the art, notably in conditions of emulsionpolymerisation. In particular, the reaction can be carried out in thepresence of a crosslinking agent, such as divinyl benzene or ethyleneglycol dimethacrylate. The crosslinking agent can represent 0 to 5 wt %,notably 0.01 to 3 wt %, of the total weight of the rheological agent.

The rheological agent can thus be in the form of a crosslinked polymer.

According to a particular embodiment, the rheological agent is in theform of a statistical copolymer, optionally crosslinked.

The term “statistical copolymer”, as used in the present invention,refers to a copolymer in which the sequence of monomer residues followsa statistical rule.

The polymerization reaction can be carried out in the presence ofanother surfactant which can be a surfactant commonly used in anemulsion polymerization reaction.

The present invention relates also to a rheological agent obtainable bythe above mentioned method.

The rheological agent can be in the form of a latex or a powder.

The present invention relates also to an oil-based drilling fluid, alsocalled oil-based drilling mud, comprising a rheological agent as definedabove.

The oil-based drilling fluids according to the invention compriseadvantageously an invert emulsion, i.e. a water-in-oil emulsioncomprising an external continuous oil phase in which is dispersed adiscontinuous internal aqueous phase.

The oil contained in the oil phase can be mineral, synthetic or diesel.The synthetic oil can be for example paraffin oils, iso-olefins,polyolefins, siloxane derivatives, etc.

The aqueous phase can be water or a brine, i.e. an aqueous solution of asalt, such as an halide or an alkali or alkaline earth metal, inparticular sodium chloride or calcium chloride.

The oil-based drilling fluids can comprise also at least one additivechosen among emulsifiers, weighting agents, wetting agents, fluid losscontrol additives, alkalinity regulators and rheology modifiers.

The emulsifier can include rosin acids, tall oil acids, fatty alcohols,or fatty acids and salts and derivatives thereof (such as amidoamines,polyamides, polyamines, esters, imidaxolines, sulfates andphosphonates), and synthetic emulsifiers. It can be in particular anemulsifier commonly used in an emulsion polymerization reaction.

The weighting agent can be barite, barium sulfate, iron oxide, galena,siderite, magnetite, illmenite, celestite, dolomite, calcite, hematiteor calcium carbonate. Preferably, it is barite or barium sulfate.

The wetting agent can be lecithin, fatty acids, tall oil, oxidized talloil, organic phosphate esters, modified imidazolines, modifiedamido-amines, alkyl aromatic sulfates, alkyl aromatic sulfonates, ororganic esters of polyhydric alcohols.

The fluid loss control additive can be lignites, asphaltic compounds,gilsonite, organophilic humates, or synthetic polymers such as Pliolite®DF.

The alkalinity regulator can be lime (calcium hydroxide) to bind orreact with acidic gases such as CO₂ and H₂S.

The rheology modifiers other than organoclays can be polyamide resins,polycarboxylic acids and fatty acids soaps.

Preferably, the oil-based drilling fluids according to the inventionwill be free of organoclays.

The oil-based drilling fluid according to the invention will containadvantageously 0.1 to 10 ppb (pounds per barrel) (0.285 to 28.5 kg/m³)of the rheological agent.

The present invention relates also to the use of a rheological agentaccording to the invention as a thixotropic agent, i.e. an agent whichforms a gel upon standing and has a low viscosity under a high shearrate, in an oil-based drilling fluid.

The rheological agent according to the invention can also be used as afluid-loss-control contributor.

Finally, the present invention relates also to a method of drillingusing an oil-based drilling fluid containing a rheological agentaccording to the invention.

The following examples are presented only to illustrate the presentinvention and are not limitative.

EXAMPLES I. Preparation of the Rheological Agent

The rheological agents according to the invention have been prepared byemulsion polymerization of monomers (a) and (b) (if present) andcopolymerisable and/or non polymerisable surfactants, optionally in thepresence of a crosslinking agent such as divinyl benzene. The monomer(s)and surfactant(s) used are as follows:

Copolymerisable Non polymerisable N^(o) Monomers (a) and optionally (b)surfactant surfactant  1* PTBS/IBOMA in wt ratio 75/25 — Dowfax ® 2A1** 2 PTBS/IBOMA in wt ratio 75/25 — Emulsogen ® EPA 073  3 PTBS/IBOMA inwt ratio 75/25 Emulsogen ® APS 100 Emulsogen ® EPA 073  4 PTBS/IBOMA inwt ratio 75/25 Reasoap ® ER-10 Emulsogen ® EPA 073  5 PTBS/IBOMA in wtratio 75/25 Reasoap ® SR-10 Emulsogen ® EPA 073  6 PTBS/IBOMA in wtratio 75/25 Reasoap ® SR-20 Emulsogen ® EPA 073  7 PTBS/IBOMA in wtratio 75/25 Reasoap ® PP-70 Emulsogen ® EPA 073  8 PTBS/IBOMA in wtratio 75/25 Emulsogen ® APS 100 Empicol ® LX 28/D***  9 PTBS/IBOMA in wtratio 75/25 Emulsogen ® APS 100 Emulsogen ® LA 083 10 PTBS/IBOMA in wtratio 75/25 Emulsogen ® APS 100 Rhodafac ® RS-610 A25 11 PTBS/IBOMA inwt ratio 75/25 Emulsogen ® APS 100 Disponil ® FES 993 12 PTBS/IBOMA inwt ratio 75/25 Reasoap ® SR-10 — Reasoap ® PP-70 13 PTBS/IBOMA in wtratio 50/50 Emulsogen ® APS 100 Emulsogen ® EPA 073 14 PTBS/PMS in wtratio 50/50 Emulsogen ® APS 100 Emulsogen ® EPA 073 15 PTBS Emulsogen ®APS 100 Emulsogen ® EPA 073 16 PTBS/IBOA in wt ratio 75/25 Emulsogen ®APS 100 Emulsogen ® EPA 073 *Comparative example **Alkyldiphenyloxidedisulfonate (it is not a non polymerisable surfactant according to theinvention) ***Sodium dodecyl sulfate (it is not a non polymerisablesurfactant according to the invention) All these examples contain about4 wt % of surfactant on a dry basis.

The rheological agents were obtained in a latex form and were used inthis form in the preparation of oil-based drilling fluids but could beused in a powder form.

II. Preparation of the Oil-Based Drilling Fluid

The rheological agents prepared in previous example I have then beenadded to the following composition A, which was then hot rolled 16 h at300° F. (149° C.), to give the oil-based drilling fluids according tothe invention.

Ingredient Activity Composition A Synthetic isomerized olefin Oil phase137 ppb  Polyaminated fatty acid Emulsifier 10 ppb Pliolite ® DFFiltration control agent  2 ppb Lime (Ca(OH)₂) Alkalinity modulator  1ppb 25% CaCl₂ brine Aqueous phase 62 ppb Finely ground altered calciumArtificail drill solids 20 ppb montmorillonite clay Barite Weightingagent 440 ppb 

III. Thixotropic Effect

The rheological profiles of the various oil-based drilling fluids testedin this part have been determined according to the API (Americanpetroleum Institute) 13A norm.

The rheological agents according to the invention have been comparedwith an organoclay and the comparative rheological agent 1, when addedto the above-mentioned oil-based drilling fluid composition A.

The following abbreviations have been used in this part:

-   -   ES@RTC: Electrical Stability at Room temperature    -   H.R.: Hot Rolling    -   HTHP: High temperature High Pressure (mL)    -   PV: Plastic Viscosity (cP)    -   YP: Yield Point (lb/100 ft²)

The results obtained are presented in the tables below:

TABLE 1 Comparison of rheological agent 2 according to the inventionwith organoclay and comparative rheological agent 1 optionally withEmulsogen ® EPA 073 A + 0.44 ppb* A + 1.0 ppb A + 0.44 of 1 + Drillingfluid organoclay ppb* of 1 EPA073** A + 0.35 ppb* of 2 Before ageingFann ® 35 dial readings at 122° F. (50° C.) (cP) 600 rpm 76 76 75 88 300rpm 44 44 43 52 200 rpm 33 33 32 40 100 rpm 22 21 21 26  6 rpm 7 6 6 10 3 rpm 6 5 5 9 Plastic viscosity (cP) 32 32 32 36 Yield point (lb/100ft²) 12 12 11 16 After ageing 16 hrs at 300° F. (149° C.) Fann ® 35 dialreadings at 122° F. (50° C.) (cP) 600 rpm 110 90 90 108 300 rpm 66 50 4961 200 rpm 49 35 35 45 100 rpm 32 21 21 28  6 rpm 11 5 5 7  3 rpm 10 4.54 6 Plastic viscosity (cP) 44 40 41 47 Yield point (lb/100 ft²) 22 10 814 HTHP at 300° F. (149° C.)/ 24.0 26.0 26.0 19.2 500 psi/2 × vol 30′(ml) *on a dry basis of the rheological agent which is introduced in alatex form **in the same amount as in rheological agent 2

These results demonstrate the importance to introduce the nonpolymerisable surfactant with monomers (a) and optionally (b) during theemulsion polymerization step and not after this step to obtain animproved rheological agent.

TABLE 2 Thixotropic effects obtained with rheological agents 3, 8 and 12according to the invention A + 1 ppb* A + 1 ppb* Drilling fluid of 3 A +1 ppb* of 8 of 12 Before ageing Fann ® 35 dial readings at 122° F. (50°C.) (cP) 600 rpm 124 127 138 300 rpm 78 84 92 200 rpm 60 66 72 100 rpm42 48 50  6 rpm 21 28 21  3 rpm 19 26 19 Plastic viscosity (cP) 46 43 46Yield point (lb/100 ft²) 32 41 36 After ageing 16 hrs at 300° F. (149°C.) Fann ® 35 dial readings at 122° F. (50° C.) (cP) 600 rpm 159 176 152300 rpm 105 122 101 200 rpm 84 101 80 100 rpm 59 75 56  6 rpm 24 30 23 3 rpm 21 26 21 Plastic viscosity (cP) 54 54 51 Yield point (lb/100 ft²)51 68 50 *on a dry basis of the rheological agent which is introduced ina latex form

TABLE 3 Thixotropic effects obtained with rheological agents 3, 4 and 6according to the invention A + 0.39 ppb* A + 0.39 ppb* A + 0.39 ppb*Drilling fluid of 3 of 4 of 6 Before ageing Fann ® 35 dial readings at122° F. (50° C.) (cP) 600 rpm 95 100 103 300 rpm 58 62 64 200 rpm 44 4950 100 rpm 30 34 34  6 rpm 13 13 13  3 rpm 12 12 12 Plastic viscosity(cP) 37 38 39 Yield point (lb/100 ft²) 21 24 25 After ageing 16 hrs at300° F. (149° C.) Fann ® 35 dial readings at 122° F. (50° C.) (cP) 600rpm 114 109 123 300 rpm 68 66 74 200 rpm 50 50 54 100 rpm 30 32 34  6rpm 10 10 11  3 rpm 8 9 9 Plastic viscosity (cP) 46 43 49 Yield point(lb/100 ft²) 22 23 25 *on a dry basis of the rheological agent which isintroduced in a latex form

The results shown on tables 2 and 3 demonstrate that a large variety ofcopolymerisable and non polymerisable surfactants can be used in therheological agents according to the invention and that other surfactantscan also be present.

TABLE 4 Thixotropic effects obtained with rheological agents 13, 3 and15 according to the invention A + 0.39 ppb* A + 0.39 ppb* A + 0.39 ppb*Drilling fluid of 13 of 3 of 15 After ageing 16 hrs at 300° F. (149° C.)Fann ® 35 dial readings at 122° F. (50° C.) (cP) 600 rpm 97 95 82 300rpm 59 58 48 200 rpm 45 44 36 100 rpm 31 30 25  6 rpm 13 13 10  3 rpm 1212 8.5 Plastic viscosity (cP) 38 37 34 Yield point (lb/100 ft²) 21 21 14Before ageing Fann ® 35 dial readings at 122° F. (50° C.) (cP) 600 rpm115 114 101 300 rpm 68 68 56 200 rpm 50 50 40 100 rpm 32 30 24  6 rpm 1010 6  3 rpm 8 8 5 Plastic viscosity (cP) 47 46 45 Yield point (lb/100ft²) 21 22 11 *on a dry basis of the rheological agent which isintroduced in a latex form

1-15. (canceled)
 16. A method to prepare a rheological agent comprisingthe polymerization of the following monomers: (a) at least onestyrene-based monomer substituted on the phenyl moiety, (b) optionallyat least one (meth)acrylate-based monomer or butadiene, and (c) acopolymerisable surfactant containing an optionally substituted vinylfunction and moieties derived from propylene oxide and/or ethyleneoxide, optionally in the presence of a non polymerisable surfactantcontaining moieties derived from propylene oxide and/or ethylene oxide,and optionally in the presence of another surfactant.
 17. The methodaccording to claim 16, wherein the copolymerisable surfactant is chosenamong anionic and non-ionic surfactants.
 18. The method according toclaim 16, wherein the non polymerisable surfactant is chosen amonganionic and non-ionic surfactants.
 19. The method according to claim 16,wherein the styrene-based monomer is styrene substituted on the phenylmoiety with one or more substituents chosen among (C₁-C₁₅)alkyl,(C₂-C₆)alkenyl, (C₅-C₈)cycloalkyl, aryl and aryl-(C₁-C₆)alkyl.
 20. Themethod according to claim 19, wherein the styrene-based monomer is a(C₁-C₆)alkyl-styrene.
 21. The method according to claim 19, wherein thestyrene-based monomer is chosen among para-tert-butyl-styrene (PTBS),para-methyl-styrene (PMS) and mixtures thereof.
 22. The method accordingclaim 16, wherein monomer (b) is present and is at least one(meth)acrylate-based monomer corresponding to the following formula:

wherein: R_(a) represents a hydrogen atom or a methyl group, and R_(b)represents a (C₁-C₁₅) saturated hydrocarbon group.
 23. The methodaccording to claim 22, wherein monomer (b) is at least one(meth)acrylate-based monomer chosen among isobornyl methacrylate(IBOMA), isobornyl acrylate (IBOA), isobutyl methacrylate (IBMA),2-ethylhexyl acrylate (2-EHA) and mixtures thereof.
 24. The methodaccording to claim 16, wherein the ratio by weight of monomer (a) tomonomer (b) is comprised between 100:0 and 10:90.
 25. The methodaccording to claim 24, wherein the ratio by weight of monomer (a) tomonomer (b) is comprised between 100:0 and 50:50.
 26. The methodaccording to claim 25, wherein the ratio by weight of monomer (a) tomonomer (b) is comprised between 100:0 and 70:30.
 27. The methodaccording to claim 16, wherein the copolymerisable surfactant has thefollowing formula:CHR′═CRL-(O—CHR^(7n)—CHR^(8n))_(n)—X¹ wherein: L represents a singlebond or a linker, R and R′ represent, independently of each other, H ora (C₁-C₂₅)alkyl, n represents an integer of at least 1, each R^(7n) andeach R^(8n) represent, independently of each other, a hydrogen atom or amethyl group but R^(7n) and R^(8n) cannot both represent a methyl group,and X¹ represents an —O-(optionally substituted aryl), —O(C₁-C₂₅)alkyl,—O—(C₁-C₆)alkanediyl-SO₃H, —OH, —S(O)₂(OH), —O—P(O)(OH)₂ or —O—S(O)₂(OH)group, or a salt thereof.
 28. The method according to claim 27, wherein:R and R′ represent, independently of each other, H or a (C₁-C₆)alkyl,and n represents an integer comprised between 3 and
 120. 29. The methodaccording to claim 28, wherein: R and R′ represent, independently ofeach other, H or a methyl, and n represents an integer comprised between5 and
 70. 30. The method according to claim 29, wherein: R and R′represent, independently of each other, H, and n represents an integercomprised between 5 and
 40. 31. The method according to claim 27,wherein X¹ represents an —O-(optionally substituted aryl),—O(C₁-C₂₅)alkyl, —O—(C₁-C₆)alkanediyl-SO₃H, —OH, —S(O)₂(OH),—O—P(O)(OH)₂ or —O—S(O)₂(OH) group, or a sodium, potassium or ammoniumsalt thereof.
 32. The method according to claim 16, wherein the nonpolymerisable surfactant has the following formula:X³—(OCH₂CH₂)_(q)—X² wherein: q represents an integer comprised between 1and 60, X³ represents a (C₈-C₂₅)alkyl chain or an aryl optionallysubstituted with a (C₁-C₂₅)alkyl, and X² represents an —OH, —S(O)₂(OH),—O—P(O)(OH)₂ or —O—S(O)₂(OH) group, or a salt thereof,
 33. The methodaccording to claim 32, wherein represents an integer comprised between 3and
 20. 34. The method according to claim 32, wherein X² represents an—OH, —S(O)₂(OH), —O—P(O)(OH)₂ or —O—S(O)₂(OH) group, or a sodium,potassium or ammonium salt thereof.
 35. The method according to claim16, wherein the polymerization is carried out in the presence of acrosslinking agent.
 36. The method according to claim 16, using 0.5 to10 wt % of copolymerisable and non polymerisable surfactants relativelyto the total weight of the rheological agent on a dry basis.
 37. Themethod according to claim 36, using 1 to 7 wt % of copolymerisable andnon polymerisable surfactants relatively to the total weight of therheological agent on a dry basis.
 38. A rheological agent obtainable bythe method according to claim
 16. 39. An oil-based drilling fluidcomprising a rheological agent according to claim
 38. 40. The oil-baseddrilling fluid according to claim 39, containing 0.1 to 10 ppb (0.285 to28.5 kg/m³) of the rheological agent.
 41. A method of drilling using anoil-based drilling fluid according to claim 39.